Magnetic recording media has begun to incorporate perpendicular magnetic recording (PMR) technology in an effort to increase areal density and has recently demonstrated densities of 612 Gbits/in2. Generally, PMR media may be partitioned into three functional regions: a soft magnetic underlayer (SUL), a nonmagnetic intermediate layer (interlayer) and a magnetic recording layer (RL). Well-isolated smaller grains of higher magnetic anisotropy constant (Ku) for a bottom magnetic recording layer can reduce media noise to achieve these higher areal densities. Enhanced grain isolation in a bottom magnetic recording layer of a PMR media structure, for example, can provide a smaller magnetic cluster size and narrow the size distribution.
It has been determined experimentally that the interlayer thickness of recording media shows a strong correlation with receiver overwrite. Since 2000, constant efforts have been made by media companies and the magnetic recording community on reducing the interlayer thickness. Today the thickness of interlayers (e.g., containing Ru) can be reduced to around 10-15 nm from a previous level of around 30-40 nm. Nevertheless, experimental data also shows that a further reduction of Ru interlayer thickness may also have a negative impact on bit error rate (BER) performance due to the resultant c-axis dispersion in recording layers. Since 2005, numerous attempts have unsuccessfully been made to fabricate interlayers with either high permeability or ultra-thin Ru thickness.
The interlayer has several key functionalities, including introducing the magnetic layer's vertical crystal growth or textural growth so as to tightly control the sigma of the magnetic grains' c-axis distribution, thereby leading to a narrow switching field distribution. Additionally, the grain size and surface morphology of the interlayer directly dictates the grain size and the grain decoupling in the recording layer. Moreover, the interlayer thickness directly impacts the head/media separation. It is well known that Ru has the same crystal structure and very similar crystal lattice parameters as CoCrPtX alloy. The use of an Ru interlayer with sufficient thickness (e.g., >10 nm) may lead to a narrow c-axis distribution and a better controlling of its dome shape surface morphology.
In cases where the interlayer is very thin or the grain size is small, it may be impossible to obtain a very small c-axis dispersion of magnetic grains in the recording layer. Since 2005, there have been various attempts to make a multi-layer interlayer by adopting a laminated magnetic or non-magnetic interlayer structure (e.g., a CoCr(10 nm)/Ru(4.5 nm) and CoPt or CoIr(6 nm)/Ru(5 nm) stack), in order to reduce the effective thickness of the interlayer while achieving similar c-axis dispersion.
In view of the above, it is a challenging task to drastically reduce Ru thickness (or to replace Ru) without increasing the switching field distribution of the recording layer. More particularly, it is extremely difficult to maintain a narrow c-axis dispersion in a small grain size setting without employing a Ru interlayer having sufficient thickness.